We wish to investigate the possible connections between the molecular basis of drug resistance and the regulation of programmed cell death. Among several mechanisms involved, we have chosen to compare the effects due to the expression of mdr genes with those caused by the expression of the protooncogene bcl-2. In order to investigate the possible interactions between these two mechanisms of drug resistance we decided, as a first step, to demonstrate that the expression in mammalian cells of the rat mdr1b gene, recently cloned in our laboratory, would provide them with the MDR phenotype. As a second step, we plan to express the human bcl-2 gene in the resistant cells containing mdr1b and in the parental control cell line, attempting (1) to compare the relative potency of these mechanisms of drug resistance, and (2) to see whether they can act synergistically with each other in protecting cells against cytotoxic agents. The first goal was achieved by subcloning the cDNA of the rat mdr1b gene from the pGEM-11Zf(+) plasmid into the EcoRI site of the pBK-CMV prokaryotic-eukaryotic expression vector, under the control of the CMV promoter. The pBK-CMV/mdr1b recombinant plasmid and the vector pBK-CMV alone were then transfected into the NIH3T3 mouse embryo fibroblasts with lipofectamine. The resistance levels conferred by mdr1b in these experiments was higher than the one shown in previous studies by using the same assay on NIH3T3 cells transfected with the human MDR1 gene under the control of the RSV promoter. Therefore, the NIH/3T3-mdr1b cells are thought to be a good model for comparing the mdr resistant phenotype with the novel bcl-2 dependent drug-resistant mechanism.